Many polymeric materials have been utilized as electrical insulating and semiconducting shield materials for power cables and other numerous applications. In order to be utilized in services or products where long term performance is desired or required, such polymeric materials, in addition to having suitable dielectric properties, must also be enduring and must substantially retain their initial properties for effective and safe performance over many years of service. For example, polymeric insulations utilized in building wire, electrical motor or machinery power wires, or underground power transmitting cables, must be enduring not only for safety but also out of economic necessity and practicality.
One major type of failure that polymeric cable sheaths can undergo is the phenomenon known as treeing. Treeing generally progresses through a dielectric section under electrical stress so that, if visible, its path looks something like a tree. Treeing may occur and progress slowly by periodic partial discharge, it may occur slowly in the presence of moisture without any partial discharge, or it may occur rapidly as the result of an impulse voltage. Trees may form at the site of a high electrical stress such as contaminants or voids in the body of the insulation-semiconductive screen interface.
Electrical treeing results from internal electrical discharges which decompose the dielectric. Although high voltage impulses can produce electrical trees, and the presence of internal voids and contaminants is undesirable, the damage which results from application of moderate A/C voltages to electrode/insulation interfaces which contain imperfections is more commercially significant. In this case, very high, localized stress gradients can exist and with sufficient time lead to initiation and growth of trees which may be followed by breakdown.
In contrast to electrical treeing, water treeing is the deterioration of a solid dielectric material which is simultaneously exposed to moisture and an electric field. It is a significant factor in determining the useful life of buried power cables. Water trees initiate from sites of high electrical stress such as rough interfaces, protruding conductive points, voids, or imbedded contaminants but at a lower field than that required for electrical trees. In contrast to electrical trees, water trees are characterized by: (a) the presence of water is essential for their growth; (b) they can grow for years before reaching a size where they may contribute to a breakdown; and (c) although slow growing they are initiated and grow in much lower electrical fields than those required for the development of electrical trees.
Electrical insulation applications are generally divided into low voltage insulation which are those less than 5K volts, medium voltage insulation which ranges from 5K volts to 60K volts, and high voltage insulation, which is for applications above 60K volts. In low voltage applications, electrical treeing is generally not a pervasive problem and is far less common than water treeing, which frequently is a problem.
For medium voltage applications, the most common polymeric insulators are made from a polyolefin, typically either from polyethylene or ethylene-propylene elastomers, otherwise known as ethylene-propylene-rubber (EPR). The polyethylene can be any one or more of a number of various polyethylenes, e.g., homopolymer, high density polyethylene (HDPE), high pressure low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and the like. The polyethylenes are typically crosslinked, usually through the action of a peroxide, but are still prone to treeing, particularly water treeing.
To counter-act this proneness to water treeing, the polymer is typically treated with a water tree-resistant agent, e.g., if the polymer is polyethylene, a typical water tree-resistant agent is polyethylene glycol. Other water tree-resistant agents are described in U.S. Pat. Nos. 4,144,202, 4,212,756, 4,263,158, 4,376,180, 4,440,671 and 5,034,278 and include, but are not limited to, organo-silanes including epoxy- or azomethine-containing organo-silanes, N-phenyl substituted amino silanes, and hydrocarbon-substituted diphenyl amines. These agents are usually mixed with the polymer before a crosslinking agent is added and before the polymer is extruded onto a cable. This mixing is typically performed as a melt blend of polymer and agent from which a pellet or other shape is formed. These blend techniques, however, are capital and/or time intensive and if the polymer is solid and the agent is liquid, do not always produce a uniform dispersion of the agent in the polymer.